Just to be sure we're completely on the same "page". Firstly, I assume you are referring to the preference Editors -> Text Editors -> Displayed tab width, correct?

Do project A and project B have associated Oomph setups?

I ask that because of course a project setup can set this preference; but I think I was wrong earlier, i.e., I think that the user setup's preference task will override the one in the project even if the project has a task for that preference. I also ask because in general, via the "Show Advanced Properties" filter in the properties view, every task has a Restrictions reference that can be used to restrict that task to perform only when that scope is present, so it's possible to define a preference that applies only for project A or project B assuming those have setups defined...